The case went before the Tokyo District Court in March, with the Naka standing accused of acquiring stock in Japanese developers Aiming and ATeam before it was publicly announced that both studios were working on projects for his then-employer, Square Enix.
In the first trial on March 2nd, Naka admitted to the charges – but in a recent update, it appears that he is now trying to pin at least some of the blame on a secretarial mistake (thanks, @gosokkyu).
It would seem that Naka's defence is that the stock purchase wasn't pre-meditated and that him seeing information that he shouldn't have had access to led him to his eventual course of action. A flimsy argument one might suggest, but, as some have said, this may well be Naka's only option at this stage when it comes to securing a more lenient sentence.
The accusation in the trial is that Naka, once he had purchased the stock, intended to resell them at a higher price, assuming that their value would rise due to the aforementioned announcements – Dragon Quest Tact (Aiming) and Final Fantasy VII: The First Soldier (ATeam), the latter of which has since been shut down.
Naka's only game during his time with Square Enix was the critically-panned Balan Wonderworld.